Multi-socked symmetric multiprocessing (SMP) system for chip multi-threaded (CMT) processors

ABSTRACT

In one embodiment, a node comprises a plurality of processor cores, coherency control circuitry coupled to the plurality of processor cores, and at least one coherence unit coupled to the coherency control circuitry. Each processor core is configured to have a plurality of threads active and each processor core includes at least one first level cache. The coherency control circuitry is configured to manage intranode coherency among the plurality of processor cores. The coherency unit is configured to couple to an external interface of the node, and is configured to transmit and receive coherence messages on the external interface to maintain coherency with at least one other node having one or processor cores and a coherence unit. In another embodiment, a system comprises an interconnect and a plurality of nodes coupled to the interconnect.

BACKGROUND

1. Field of the Invention

This invention is related to the field of SMP and CMT, and particularlyto a system for implementing SMP with CMTs.

2. Description of the Related Art

Chip multithreading (CMT) has been proposed as one way to utilize thetransistors that can now be integrated on the same semiconductorsubstrate, or integrated circuit chip. Particularly, one or moreprocessor cores may be included on a chip, and each processor core ismultithreaded. That is, the processor core includes hardware to havemultiple threads active. Software can activate the threads forexecution, and the hardware handles executing the threads, interleavingaccess among the instructions from different threads to shared processorcore hardware, etc.

Prior to the advent of multithreading, executing more than one threadconcurrently required multiple single-threaded processors. Eachprocessor is a separate chip, and the processors are connected in somefashion to memory. For example, in the symmetric multiprocessing (SMP)model, processors are considered equals and any processor can executeany thread (or process). Typically, cache coherency (or more briefly,coherency) is maintained to ensure that memory accesses from differentprocessors occur in a coherent fashion (e.g. the most recently updatesto bytes at a given address are delivered in response to a read of thegiven address).

One SMP implementation is a multidrop bus to which each single-threadedprocessor chip is connected. An address transmitted on the bus (e.g. byone of the processors) is snooped by the other processors, which checktheir caches for a copy of the data being accessed. In otherimplementations, explicit probe transactions (or inquire transactions)are transmitted on the bus and only the probe addresses are snoopedagainst the cache.

The multidrop SMP bus is generally not suitable for handling multipleCMTs. The multidrop bus is highly capacitive, which generally means thatthe bus clock cycle is relatively long. The bandwidth and latency on thebus is thus limited. CMTs, executing multiple threads/processes and thusgenerating numerous memory accesses to many different addresses, wouldexceed the available bandwidth and the latency would reduce performance.Additionally, CMTs that include multiple processor cores would not beable to maintain coherency using the multidrop bus. Another method usedto interconnect single-threaded processors is the point-to-pointinterconnect of Hypertranspor™ and the like.

SUMMARY

In one embodiment, a node comprises a plurality of processor cores,coherency control circuitry coupled to the processor cores, and at leastone coherence unit coupled to the coherency control circuitry. Eachprocessor core is configured to have a plurality of threads active andeach processor core includes at least one first level cache. Thecoherency control circuitry is configured to manage intranode coherencyamong the processor cores. The coherency unit is configured to couple toan external interface of the node, and is configured to transmit andreceive coherence messages on the external interface to maintaincoherency with at least one other node having processor cores and acoherence unit. In another embodiment, a system comprises aninterconnect and a plurality of nodes coupled to the interconnect.

In yet another embodiment, an apparatus comprises a source node and acoherence hub. The coherence hub is configured to route coherencemessages between the source node and other nodes. The source node isconfigured to generate a node request, which is one of the coherencemessages, to initiate coherence activity for access to a given memorylocation and to assign a source identifier to the node request thatidentifies the source node and the node request. The coherence hub isconfigured to indicate that the source identifier is released forassignment to another node, wherein the coherence hub is configured toprovide the indication in another coherence message to the source nodethat is part of the coherence activity, if possible, and wherein thecoherence hub is configured to explicitly transmit a source identifierrelease message to provide the indication if the indication cannot beprovided in another coherence message that is part of the coherenceactivity.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description makes reference to the accompanyingdrawings, which are now briefly described.

FIG. 1 is a block diagram of one embodiment of a CMT node.

FIG. 2 is a block diagram of one embodiment of two CMT nodes coupledtogether.

FIG. 3 is a block diagram of one embodiment of four CMT nodes coupledtogether via external coherence hubs.

FIG. 4 is a block diagram illustrating one embodiment of mapping anaddress space to multiple coherence planes.

FIG. 5 is a block diagram of one embodiment of an address and fieldswithin the address.

FIG. 6 is a block diagram illustrating one embodiment of address bitsfrom the address shown in FIG. 5 and mapping the L2 cache banks andcoherence planes.

FIG. 7 is a block diagram of another embodiment of an address and fieldswithin the address.

FIG. 8 is a block diagram illustrating one example of one embodiment ofa coherency maintenance protocol for data that is local to the sourcenode.

FIG. 9 is a block diagram illustrating one example of one embodiment ofa coherency maintenance protocol for data that is remote to the sourcenode.

FIG. 10 is a block diagram illustrating one example of one embodiment ofa coherency maintenance protocol for a writeback from a source node.

FIG. 11 is a block diagram illustrating one embodiment of framing on aninterconnect between nodes and coherency messages.

FIG. 12 is a flowchart illustrating one embodiment of a method forcoherence planes.

FIG. 13 is a block diagram of another embodiment of CMT nodes coupledand also illustrating memory modules coupled to the nodes.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF EMBODIMENTS

Turning now to FIG. 1, a block diagram of one embodiment of a CMT node10 is shown. In one embodiment, the CMT node 10 may be a singleintegrated circuit “chip”. In other embodiments, the node may comprisetwo or more integrated circuits or other circuitry. In the illustratedembodiment, the CMP node 10 comprises one or more processor cores (ormore briefly, cores) 12A-12N, a second level (L2) cache 14, one or morememory control units (MCUs) 16, one or more coherence units (CUs) 18,and one or more I/O control units such as I/O control unit 20. The cores12A-12N, the MCUs 16, the CUs 18, and the I/O control unit 20 arecoupled to the L2 cache 14. The I/O control unit 20 is configured tocouple to one or more I/O interfaces. The MCUs 16 are configured tocoupled to one or more external memories 22. The coherence units 18 areconfigured to couple to one or more external interfaces 24 of the node10 to communicate with other nodes. In the illustrated embodiment, eachof the cores 12A-12N include one or more first level (L1) caches 26A-26N(and thus the cache 14 is an L2 cache). In the illustrated embodiment,the L2 cache 14 includes an L1 coherence control unit 28.

The coherence units 18 may be configured to communicate with coherenceunits in other nodes to maintain internode coherency for the memoryaccessible to the nodes. Data from the memory may be cached (e.g. in theL2 cache 14 and/or the L1 caches 26A-26N), and the coherence units 18may ensure coherent memory access occurs (e.g. a read of a memorylocation results in the return of the data written by the most recentwrite to that memory location). Each memory location in the memorysystem is identified by an address, and the coherence units 18 mayensure coherent access to the memory locations based on the addressesused to access those locations. Generally, a coherence unit may comprisecircuitry configured to maintain internode coherency.

In one embodiment, the coherence units 18 may be configured to provide“glueless” connection to another node 10. That is, the coherence units18 in the two nodes may be configured to communicate directly over theexternal interfaces 24 with each other to provide coherence for a twonode system. For more than two nodes, the coherence units 18 in eachnode may communicate over the external interfaces with an externalcoherence hub. The coherence hub may be responsible for orderingrequests from the nodes, for forwarding coherence messages to othernodes, and for gathering coherence messages from the nodes (respondingto coherence messages sent from the coherence hub) to ensure thecoherency of each transaction.

In one embodiment, the external interfaces 24 may leverage an industrystandard for at least part of the external interface definition. Forexample, the physical layer of communication may be leveraged. Thephysical layer may include the transmission media over which thecommunication is transmitted, as well as the circuitry used to drive thetransmission media. Additionally, in some embodiments, the framing usedon the standard interface may be leveraged. The logical definition ofthe messages transmitted in the frames may differ from the standard.More particularly, some embodiments may implement the physical layer andframing used to communicate with fully buffered dual-in line memorymodules (FBDIMMs). The FBDIMM interface uses point to point linkscomprising multiple lanes of transmission. Each lane is a serialtransmission medium, and a serializer/deserializer (SERDES) is definedfor transmitting symbols over the lane. By using the FBDIMM physicalinterface, the same SERDES design and physical links may be used.Furthermore, the framing (defining one physical transfer on the link)and related circuitry may also be used. The FBDIMM interface may becompatible, e.g., with JEDEC specifications. (Note that JEDEC wasformerly an acronym for Joint Electron Device Engineering Council, butthe name is now simply JEDEC). Other standard memory interfaces may beused in other embodiments. The external interfaces used for coherencymay be derived from the memory interface, sharing at least somecharacteristics of the memory interface.

Using the FBDIMM external interface (or another standard memoryinterface) for the coherent interfaces between the coherence units 18 ofmultiple nodes may also permit the coherent interfaces to improveperformance over time in lockstep with improvements in the memoryinterfaces. More particularly, in one embodiment, the externalinterfaces 24 may be same interface as the MCUs 16 implement tointerface to the memory 22. The coherent interfaces may thus “keep up”with the memory that may be included in the system with the node 10.

The node 10 and one or more other nodes in a system may each be coupledto memories such as the memory 22 shown in FIG. 1 to form a distributedmemory system. The memory address space of the system may be mapped overthe distributed memory system. Accordingly, a given address in a givennode may be local (addressing a memory location in the memory attachedto the given node) or remote (addressing a memory location in a memoryattached to another node). In either case, coherence activity may beneeded to perform the access coherently. Generally, coherence activitymay refer to any communication or communications between nodes to ensurethat a given memory access is coherent.

In one implementation, the node 10 may implement multiple independentcoherence planes. That is, coherence activity in one coherence plane maybe independent of coherence activity in other coherence planes. Theaddress space is divided among the coherence planes, each addressmapping to one coherence plane. The addresses within the same coherencegranule map to the same coherence plane. The coherence granule may bethe unit of memory for which coherency is maintained. That is, anyaccess or modification of a byte within the coherence granule affectsthe coherence state of the entire granule. In one embodiment, thecoherence granule is a cache line, which is the unit of allocation anddeallocation of storage space in the caches. The cache line will be usedas an example in this description, but any coherence granule (e.g. afraction or multiple of a cache line size) may be used in otherembodiments.

The mapping of addresses to coherence planes may be independent of thephysical location, within the system, of the memory location mapped tothe address. Both local and remote memory locations may be included in agiven coherence plane, as may remote memory locations that are mapped todifferent nodes. Dividing the address space into independent coherenceplanes may provide higher bandwidth for coherence messages betweennodes. Furthermore, the use of coherence planes may be scalable. Thatis, if additional bandwidth for coherence messages is needed, additionalcoherence planes may be added. A different coherence unit 18 may beprovided for each coherence plane (and separate external interfaces 24may also be provided for each coherence plane).

While coherence planes are described herein with respect to CMT nodes,coherence planes may also be used with non-CMT nodes (e.g. nodescomprising one or more single-threaded processor cores). Generally, anode may comprise any circuitry that is treated as a unit forsystem-wide coherence purposes. There may also be intranode coherency(e.g. coherency among the L1 caches 26A-26N) in some embodiments. Otherembodiments (e.g. embodiments that employ a single processor core orprocessor cores without L1 caches) may not require intranode coherency.Similarly, the use of standard memory interfaces, such as the FBDIMMinterfaces, for coherence interfaces between nodes may be employed inembodiments having non-CMT nodes.

The intranode coherency is managed, in the illustrated embodiment, bythe L1 coherence control unit 28 in the L2 cache 14. The coherencecontrol unit 28 may maintain coherence among the L1 caches 26A-26N inany desired fashion. For example, in one embodiment, the L1 coherencecontrol unit 28 may track which cache lines are stored in each L1 cache.The L2 cache 14 may be inclusive of the L1 caches 26A-26N, in oneembodiment, and the tracking may be implemented as state in the tags ofeach L2 cache block. Alternatively, the L2 cache 14 may include separatetags for the L1 cache tracking. The L2 cache 14 may perform a reverselookup on these separate tags based on the L2 index and way to read theinformation identifying which L1 caches 26A-26N have a copy of the cacheblock.

The L1 coherence control unit 28 may also respond to requests from thecoherence units 18. The coherence units 18 may provide snoop requests,invalidate requests, etc. as part of maintaining internode coherence,and the coherence control unit 28 may respond accordingly and may causestate changes in the L1 caches 26A-26N as needed. The L1 coherencecontrol unit 28 may generate communications to the L1 caches 26A-26N tomaintain intranode coherency and to respond to requests from thecoherence units 18 to maintain internode coherency. The communicationsmay include, for example, invalidate requests, requests to writebackmodified cache lines, state change requests, etc. The L1 coherencecontrol unit 28 may have any interconnect with the L1 caches 26A-26N.

The processor cores 12A-12N may be configured to execute instructionsand to process data according to a particular instruction setarchitecture (ISA). In one embodiment, cores 12A-12N may be configuredto implement the SPARC® V9 ISA, although in other embodiments it iscontemplated that any desired ISA may be employed, such as x86, PowerPC®or MIPS®, for example. In the illustrated embodiment, each of cores12A-12N may be configured to operate independently of the others, suchthat all cores 12A-12N may execute in parallel. In some embodiments,each of cores 12A-12N may be configured to execute multiple threadsconcurrently, where a given thread may include a set of instructionsthat may execute independently of instructions from another thread. (Forexample, an individual software process, such as an application, mayconsist of one or more threads that may be scheduled for execution by anoperating system.) Such a core may also be referred to as amultithreaded (MT) core. In one embodiment, there may be 8 cores12A-12N, each of which may be configured to concurrently executeinstructions from eight threads, for a total of 64 threads concurrentlyexecuting across CMT 10. However, in other embodiments it iscontemplated that other numbers of cores 12A-12N (including one core)may be provided, and that cores 12A-12N may concurrently processdifferent numbers of threads. A thread may be referred to as “active” ifthe thread is in execution in a core, even if none of the instructionsfrom that thread are being executed at that point in time. Viewed inanother way, a thread may be active if at least a portion of the contextof the thread is being maintained in a core and the core may fetch andexecute instructions from the thread without software intervention. Incontract, inactive threads may be in memory or elsewhere (e.g. paged todisk) awaiting scheduling to a core.

More specifically, in one embodiment each of cores 12A-12N may beconfigured to perform fine-grained multithreading, in which each coremay select instructions to execute from among a pool of instructionscorresponding to multiple threads, such that instructions from differentthreads may be scheduled to execute adjacently. For example, in apipelined embodiment of a core 12A-12N employing fine-grainedmultithreading, instructions from different threads may occupy adjacentpipeline stages, such that instructions from several threads may be invarious stages of execution during a given core processing cycle.

In the illustrated embodiment, the cores 12A-12N include L1 caches26A-26N. Any cache configuration and capacity may be implemented. Forexample, in one embodiment, separate L1 instruction and data caches maybe implemented to store instructions for core execution and dataoperated upon by the core, respectively. Other embodiments may have ashared instruction/data cache.

The L2 cache 14 may be configured to cache instructions and data for useby cores 12A-12N. In one embodiment, the L2 cache 14 may be organizedinto eight separately addressable banks that may each be independentlyaccessed, such that in the absence of conflicts, each bank mayconcurrently return data to a respective core 12A-12N. In someembodiments, each individual bank may be implemented usingset-associative or direct-mapped techniques. For example, in oneembodiment, the L2 cache 14 may be a 4 megabyte (MB) cache, where each512 kilobyte (KB) bank is 16-way set associative with a 64-byte cacheline size, although other cache sizes and geometries are possible andcontemplated. The L2 cache 14 may be implemented in some embodiments asa writeback cache in which written (dirty) data may not be written tosystem memory until a corresponding cache line is evicted. Otherembodiments may implement other numbers of banks.

The MCUs 16 may be configured to manage the transfer of data between theL2 cache 14 and the memory 22. In some embodiments, multiple instancesof an MCU may be implemented, with each instance configured to control arespective bank of system memory. The MCUs 16 may be configured tointerface to any suitable type of system memory, such as FBDIMMs, DoubleData Rate or Double Data Rate 2 Synchronous Dynamic Random Access Memory(DDR/DDR2 SDRAM), or Rambus® DRAM (RDRAM®), for example. In someembodiments, the MCUs 16 may be configured to support interfacing tomultiple different types of system memory.

The I/O control unit 20 may be configured to provide a central interfacefor various input/output and/or peripheral devices to exchange data withthe cores 12A-12N through memory locations (e.g. cached in the L2 cache14 or in the memory 22). In some embodiments, the I/O control unit 20may be configured to coordinate Direct Memory Access (DMA) transfers ofdata between various devices coupled to one or more I/O interfaces suchas network interfaces and/or peripheral interfaces and system memory. Inaddition, in one embodiment, the I/O control unit 20 may be configuredto couple CMT node 10 to external boot and/or service devices. The I/Ointerfaces coupled to the I/O control unit 20 may include one or moreperipheral interfaces configured provide connection for one or moreperipheral devices. Such peripheral devices may include, withoutlimitation, storage devices (e.g., magnetic or optical media-basedstorage devices including hard drives, tape drives, CD drives, DVDdrives, etc.), display devices (e.g., graphics subsystems), multimediadevices (e.g., audio processing subsystems), or any other suitable typeof peripheral device. In one embodiment, the peripheral interface mayimplement one or more instances of an interface such as PeripheralComponent Interface Express (PCI Express™), although it is contemplatedthat any suitable interface standard or combination of standards may beemployed. For example, in some embodiments peripheral interface 150 maybe configured to implement a version of Universal Serial Bus (USB)protocol, IEEE 1394 (Firewire) protocol, PCI, etc. in addition to orinstead of PCI Express . The I/O interfaces may also include one or morenetwork interfaces to couple the CMT node 10 to a network. In oneembodiment, the network interface may be an Ethernet (IEEE 802.3)networking standard such as Gigabit Ethernet or 10-Gigabit Ethernet, forexample, although it is contemplated that any suitable networkingstandard may be implemented. In some embodiments, the network interfacemay be configured to implement multiple discrete network interfaceports.

Turning now to FIG. 2, a block diagram is shown of one embodiment of asystem employing two CMT nodes 10A and 10B, each of which may be aninstantiation of the node 10 shown in FIG. 1. The CMT node 10A includesa crossbar (XBar) 30, the L2 cache 14, coherence units 18A-18D (each ofwhich may be an instantiation of a coherence unit 18 as shown in FIG.1), and memory control units 16A-16B (each of which may be aninstantiation of a memory control unit 16 as shown in FIG. 1). The node10B similarly includes coherence units 18E-18H and memory control units16C-16D (and may also include the L2 cache 14, etc., similar to FIG. 1and not shown in FIG. 2). The crossbar 30 is coupled to communicateto/from the cores 12A-12N and the I/O control unit 20, and is coupled tothe L2 cache 14. The coherence units 18A-18D and the memory controlunits 16A-16B are coupled to the L2 cache 14 as well. The coherenceunits 18A-18D are each coupled to a respective external interface24A-24D (each of which is an instantiation of the interface 24 shown inFIG. 1) to which coherence units 18E-18H are respectively coupled asshown in FIG. 2. The coherence unit 18A is shown in more detail toinclude a coherence hub (CH) 40 and a link interface unit (L1 U) 42, andthe coherence units 18B-18H may be similar. The memory control units16A-16D are coupled to respective memories 22A-22D (each of which may bean instantiation of the memory 22 shown in FIG. 1). Collectively, thememories 22A-22D form a distributed memory system over which the memoryaddress space of the system is distributed.

More particularly, the L2 cache 14 in the illustrated embodimentcomprises M banks shown as bank 0 to bank M-1 in FIG. 2 (M is aninteger). The number of banks may be varied from embodiment toembodiment. In one implementation, the number of banks may be greaterthan or equal to the number of cores 12A-12N, providing high bandwidthcache access to the cores 12A-12N. In one particular implementation, 8banks may be used, although any number of banks may be implemented invarious embodiments. Bank 0 is shown in greater detail to include an L2tags memory 32, an L2 data memory 34, a control unit 36, and an L1 tagsmemory 38. Other banks 1 to M-1 may be similar to bank 0.

Each cache line is mapped, according to its address, to one of the banks0 to M-1. Accordingly, the crossbar 30 may include logic (such asmultiplexers or a switch fabric, for example) that allows any core12A-12N to access any bank 0 to M-1 of the L2 cache 14, and thatconversely allows data to be returned from any L2 bank 0 to M-1 to anycore 12A-12N. The crossbar 30 may be configured to concurrently processdata requests from the cores 12A-12N to the L2 cache 14 as well as dataresponses from the L2 cache 14 to the cores 12A-12N. In someembodiments, the crossbar 30 may also include logic to queue datarequests and/or responses, such that requests and responses may notblock other activity while waiting for service. Additionally, in oneembodiment, the crossbar 30 may be configured to arbitrate conflictsthat may occur when multiple cores 12A-12N attempt to access a singlebank of the L2 cache 14 or multiple banks attempt to return dataresponses to a single core 12A-12N.

The L2 tags memory 32 may store the tags of cache lines currently storedin bank 0, along with various status information. The status informationmay include whether or not the cache line is valid, along with itscoherency state according to the internode coherency scheme implementedin the system. For example, the Modified, Exclusive, Shared, Invalid(MESI) scheme may be used or the MOESI scheme (including the MESI statesand the Owned state) may be used. Variations of the MESI and/or MOESIschemes may be used, or any other scheme may be used. In response to arequest from a core 12A-12N or the I/O control unit 20, if the addressis a hit in the L2 tags memory 32 but the coherency state does notpermit the completion of the request, the control unit 36 may generate acoherence message to the coherence unit 18A to obtain the appropriatecoherence state. The coherence message may include the address, as wellas the type of request. Similarly, if the address is a miss in the L2tags memory 32, the control unit 36 may generate a coherence message tothe coherence unit 18A to obtain a copy of the cache line in theappropriate coherence state. If the coherency state does permit thecompletion of the request, the control unit 36 may complete the request(updating the L2 data memory 34 and/or supplying data from the L2 datamemory 34).

Additionally, the control unit 36 may track which cache lines are storedin which L1 caches using the L1 tags memory 38. In one embodiment, theL1 tags memory 38 includes a location for each tag in the L2 tags memory32. The L1 tags memory 38 may be indexed by the L2 tag, and may storedata that identifies which L1 caches, if any, are storing a copy of thecache line. The control unit 36 may consult the L1 tags memory 38 whenpermitting a request to complete, to manage the coherency of the L1caches. That is, if a state change in one L1 cache in one core is neededto permit the request from another core to complete, the control unit 36may generate the communication to that core/L1 cache to cause the statechange. Additionally, the control unit 36 may update the L1 tags memory38 to reflect completion of the request (e.g. indicating that the L1cache has a copy of the cache line, that the cache line is modified inthat L1 cache if the request is a write, etc.). Accordingly, the controlunit 36 (or at least the portion that interacts the L1 tags memory 38and controls coherency) may form a portion of the L1 coherence controlunit 28 shown in FIG. 1, along with similar portions of the other banks1 to M-1.

Each of the coherence units 18A-18D may correspond to a differentcoherence plane, as mentioned above. In the illustrated embodiment, eachof the banks is coupled to one of the coherence units 18A-18D, and notto the other coherence units 18A-18D. Thus, addresses that map to agiven bank also belong to the coherence plane corresponding to thecoherence unit 18A-18D to which the given bank is coupled. For example,in the illustrated embodiment, pairs of banks are coupled to eachcoherence unit 18A-18D. Specifically, banks 0 and 1 are coupled to thecoherence unit 18A; banks 2 and 3 are coupled to the coherence unit 18B;banks M-4 and M-3 are coupled to the coherence unit 18C; and banks M-2and M-1 are coupled to the coherence unit 18D. In other embodiments, asingle bank may be coupled to each coherence unit, or more than twobanks may be coupled to a given coherence unit. Still other embodimentsmay map addresses to coherence units/planes in other fashions. In oneimplementation, pairs of banks are mapped to coherence units and thereare 8 banks, so there are 4 coherence units/planes. Otherimplementations may implement other numbers of banks and coherenceplanes.

Since banks are assigned to specific coherence planes in thisembodiment, data and other responses returned from the externalinterfaces 24A-24D may be driven to the banks assigned to thecorresponding coherence plane. Serialization of data transfers from thecoherence units 18A-18D across all the banks, complex interconnect suchas the crossbar 30, etc. may not be required. For example, in theillustrated embodiment, data returned for a request on one of thecoherence planes need only be driven to the pair of banks correspondingto that coherence plane, and one of the pair may be enabled to write thedata. A high speed cache to cache transfer from another node's L2 to theL2 cache 14 in the node 10A may thus be supported, in some embodiments.

A given bank may store data for both local and remote addresses, andthus local and remote addresses may be mixed in the same coherenceplane. Even if an address is local, coherency activity may be needed(e.g. to invalidate remotely cached copies of the data, to obtain aremotely cached and updated copy of the data, etc.).

Each of the coherency units 18A-18D operates independently, over itscorresponding interface 24A-24D, to maintain coherency for addresseswithin its corresponding coherence plane. That is, the coherencyactivity of each coherency unit 18A-18D may not impact the coherencyactivity of the other coherency units, either logically or physically.In the illustrated embodiment, each coherency unit 18A-18D maycommunicate directly with the corresponding coherency unit 18E-18H,respectively, in the node 10B. That is, the coherency units 18A and 18E,for example, cooperate directly and exchange coherence messages over theinterface 24A to ensure the coherency of a given cache line. Theindependence of coherence planes/coherence units may provide scalabilityfor coherence bandwidth between nodes. Bandwidth may be increased byadding coherence planes to the address space. In some multi-threadedembodiments, the scalability provided by the coherence planes may helpprovide the higher memory bandwidth requirements often exhibited bymulti-threaded execution.

As illustrated for the coherence unit 18A in FIG. 2, each of thecoherence units 18A-18H may implement a coherence hub 40 that may serveas the point of serialization and global ordering for the correspondingcoherence plane. That is, the order that requests arrive at thecoherence hub 40 is the order for coherency purposes. The coherence hub40 may receive requests corresponding to the coherence plane, serializeand order the requests, generate coherence messages to nodes other thana source node for a request and collect responses from those othersource nodes, and communicate completion of the coherence activity tothe source node. An exemplary coherence protocol and the messages usedare described in more detail below with regard to FIGS. 8, 9, and 10.

As mentioned previously, the interfaces 24 may leverage a standardinterface for physical layer and framing. For example, the FBDIMMinterface may be used, and may be implemented by the memory controlunits 16A-16D as well to communicate with the memories 22A-22D. In oneimplementation, the FBDIMM memory interfaces implemented by the memorycontrol units 16A-16D may include synchronous unidirectional point topoint links. The northbound (from memory to the memory control unit)link may be 14 lanes wide and the southbound (from the memory controlunit to the memory) link may be 12 lanes wide. A frame may be 12transfers on the lanes. The widths and frame size may be changed fromtime to time as the FBDIMM interface standard evolves. In oneembodiment, the external interfaces 24 may comprise FBDIMM interconnectthat is 14 lanes wide in both directions, with a frame size of 12transfers, for 168 bits per frame. That is, the external interfaces 24may comprise FBDIMM links similar to the northbound links of theinterfaces to the memories 22.

The link interface unit 42 may implement the physical layer and framingof the transmissions on the interface 24A. Thus, the link interface unit42 may use a standard design for the FBDIMM interconnect, in oneembodiment. The existing design may be leveraged to save design time andcost.

In the illustrated embodiment, the memory control units 16A-16B may alsobe coupled to specific banks 0 to M-1 and not coupled to other banks.The memory control units 16C-16D may be coupled similarly to L2 banks inthe node 10B (not shown in FIG. 2). For example, the memory control unit16A in FIG. 2 is coupled to banks 0 through 3, and the memory controlunit 16B is coupled to banks M-4 through M-1. Each memory control unitis thus coupled to a quad (two pairs) of banks, corresponding to twocoherence planes. In the present embodiment, two FBDIMM interfacesprovided between each memory control unit 16A-16D and the correspondingmemory 22A-22D. Providing two FBDIMM interfaces may permit 128 bits ofdata plus 16 bits of ECC and chip kill support in the memory controlunits 16A-16D. Other embodiments may have one FBDIMM interface or morethan two. Furthermore, the number of banks coupled to a given memorycontrol unit may be varied in other embodiments, as may the number ofmemory control units per node.

While the control unit 36 (and more generally the L2 cache 14) generatescoherence messages to maintain internode coherency for a request in thepresent embodiment, other embodiments may not include an L2 cache andmay generate the coherence messages from any source. Generally,circuitry within the node 10A-10B may be configured to generatecoherence messages to initiate intranode coherency activity for a givenrequest.

FIG. 3 is a block diagram of another embodiment of a system comprising 4CMT nodes 10A-10D, each of which may be an instantiation of the node 10shown in FIG. 1. Illustrated in FIG. 3 is the L2 cache 14 in each node10A-10D, with banks 0 to M-1. Each pair of banks is coupled to acorresponding coherence unit. For example, banks 0 and 1 are coupled tocoherence unit 18A and banks M-2 and M-1 are coupled to the coherenceunit 18D in node 10A. Nodes 10B-10D similarly include coherence units18E and 18H-18L coupled to banks 0 and 1 or M-2 and M-1 as shown in FIG.3. Each coherence unit is coupled to an interface (e.g. interfaces 24Aand 24D-24J as shown in FIG. 3. Also illustrated are external coherencehubs 50A and 50B. There may be one coherence hub for each coherenceplane. The interfaces coupled to coherence units for a given coherenceplane in each node are coupled to the corresponding coherence hub. Forexample, the interfaces 24A, 24E, 24H, and 24J are coupled to thecoherence units 18A, 18E, 18J, and 18L, all of which are coupled tobanks 0 and 1 of the L2 cache 14 in their respective nodes. Theinterfaces 24A, 24E, 24H, and 24J are also coupled to the coherence hub50A. The coherence hub 50A is the point or serialization and global(internode) ordering for the coherence plane corresponding to banks 0and 1. Similarly, the coherence hub 50B is the coherence hub for thecoherence plane corresponding to banks M-2 and M-1. Other coherencehubs, not shown in FIG. 3, correspond to other coherence planes.

When the external coherence hubs are in use, such as the system of FIG.3, the internal coherence hubs 40 may be disabled. The link interfaceunits 42 may still be used to transmit the coherence messages to theexternal coherence hubs. While four nodes 10A-10D are shown in FIG. 3,other embodiments may have any number of three or more nodes withexternal coherence hubs. In still other embodiments, internal coherencehubs may not be implemented and external coherence hubs may be used withany number of two or more nodes.

The mapping of addresses to L2 cache banks, and thus the mapping ofaddresses to coherence planes, may be fixed or programmable, in variousembodiments. Generally, the coherence planes may be interleavedthroughout the address space as illustrated in FIG. 4. FIG. 4 is a blockdiagram illustrating an address space divided into coherence planes forone embodiment. Address 0 is depicted at the top, and addresses increasetoward the bottom of the address space as shown in the figure. The firstfew addresses are mapped to coherence plane 0 (CP0) (reference numeral60), followed by addresses mapped to coherence planes 1, 2, and 3 inturn (reference numerals 62, 64, and 66), and then returning tocoherence plane 0 (reference numeral 68). The size of the interleave mayvary from embodiment to embodiment. In one embodiment, for example,pairs of L2 cache banks are mapped to the same coherence plane and thusthe interleave may occur on boundaries that are twice the cache linesize (e.g. 128 byte boundaries, for a 64 byte cache line size).

As mentioned above, the mapping of addresses to coherence planes may beindependent of the physical memory location to which the addresses aremapped in the distributed memory system. That is, a given coherenceplane may include a mix of local and remote addresses, and remoteaddresses mapped to memory attached to different remote nodes. Themapping of the address space to the distributed memories may be fixed orprogrammably selectable. For example, in one embodiment, the mapping maybe programmably selected to occur on 512 byte or 1 Gigabyte boundaries.Other embodiments may include more mapping options and/or differentmapping options.

Turning next to FIG. 5, a block diagram is shown illustrating oneembodiment of fields within an address for a 512 byte boundary formapping of addresses to nodes in the distributed memory system. Theaddress may be P bits (P-1 to 0), where P is an integer. For example, 40bits may be used although more or fewer bits may be used in otherembodiments. In the embodiment of FIG. 5, the least significant 6 bits(bits 5:0) are the cache line offset since the cache line size is 64bytes in this embodiment. Bits 8:6 are the L2 bank select bits (for an 8bank L2 cache). Since pairs of banks are mapped to coherency planes,bits 8:7 also identify the coherence plane (CP) to which the address ismapped for this embodiment. Bits 10:9 select the node, for a four nodesystem, and the remaining bits are L2 index and tag bits. Bit 9 may beused to select the node for a two node system, and additional bits (e.g.bit 11, bit 12, etc.) may be used for systems having more than fournodes.

FIG. 6 is a block diagram illustrating the mapping of addresses to L2cache banks and coherence planes in a tabular form, for one embodimentusing the address fields illustrated in FIG. 5. The values for addressbits 8:6 are illustrated on the horizontal axis of the table, and thevalues for address bits P-1:9 are illustrated on the vertical axis ofthe table. For P-1:9 equal to zero (first row of the table), theaddresses are all mapped to node 0 (N0). Eight cache lines are shown (B0to B7), one for each value of bits 8:6. Pairs of cache lines map torespective coherence planes CP0 to CP3. For P-1:9 equal to one (secondrow of the table), the addresses are all mapped to node 1 (N1), etc.through node 3 for P-1:9 equal to 3. For P-1:9 equal to four, themapping is back to node 0 again, as shown in the last illustrated row ofthe table. Similar rows may exist for each additional value of P-1:9.

FIG. 7 is a block diagram illustrating one embodiment of fields withinan address for a 1 Gigabyte boundary for mapping of addresses to nodesin the distributed memory system. In the embodiment of FIG. 7, similarto the embodiment of FIG. 6, bits 5 5:0 are the cache line offset, bits8:6 are the L2 bank select bits, and bits 8:7 identify the coherenceplane. Bits 32:31 select the node, for a four node system, and theremaining bits are L2 index and tag bits.

The coherence messages and coherence protocol are described next. Forthis description, the source node may be the node that initiatescoherence activity (e.g. due to a cache miss for an address, or toupgrade ownership of the cache line to complete a request, etc.). Thecoherence hub is also shown, which is the coherence hub for thecoherence plane to which the address affected by the coherence activityis mapped. In embodiments similar to FIG. 2, the coherence hub may beintegrated into one of the nodes. In embodiments similar to FIG. 3, thecoherence hub may be external to the nodes. Other nodes are also shown.To facilitate illustrating the difference between a node that is thememory agent of a remote address from the source node and a non-memoryagent, two other nodes are shown. A memory agent is the node to whichthe address is mapped within the distributed memory system. That is, thephysical memory location in the main memory that is mapped to theaddress is in the memory attached to the memory agent node.

In one embodiment, the coherence messages may be categorized into threecategories: requests, responses, and data. The request category mayinclude the request generated by the source node to the coherence huband also forwarded requests from the coherence hub to other nodes. Theforwarded requests may be generated by the coherence hub in response tosource node requests. The response category may include a forwardedrequest acknowledgement from the coherence hub to the source nodeindicating that the request has been serialized/ordered in the hub; anode snoop reply from each node to the coherence hub indicating thenode's response to the snoop; a forwarded snoop reply from the coherencehub to the source node indicating the aggregate snoop response from allnodes; and a source identifier release message from the coherence hub tothe source node, indicating that the source ID assigned to a request maybe reused. The data category may include a node data message from thesource node to the coherence hub including the data from the source node(e.g. for write requests); a forwarded data message from the coherencehub to a target node; a node data reply including data from a snoop hitor data from a memory agent, from a node to the coherence hub; and aforwarded data reply from the coherence hub to the source node includingdata from a node that detected a snoop hit or data provided by a memoryagent.

FIG. 8 is a block diagram illustrating communications according to oneembodiment of the coherence protocol and coherence messages. In theexample of FIG. 8, the source node is also the memory agent for theaddress affected by the coherency activity. That is, the address islocal to the source node. A source node 70 is illustrated, along with acoherence hub 72 and two other nodes 74 and 76. The example of FIG. 8may represent coherence activities for various read requests (e.g. readto share, read to own, read to discard, etc.) as well as a flush requestthat causes a flush of the affected cache line (except that no data isreturned to the source node for the flush, but rather is returned to thememory agent node). While 3 nodes are shown in FIG. 8, any number of twoor more nodes may be used in various embodiments.

The source node 70 transmits a node request to initiate the coherenceactivity. The node request may include a source identifier (SID) thatmay be used to identify the various coherence messages associated withthe coherence activity for this request from among all coherencemessages being exchanged on the coherence plane. The request may alsoinclude the address, a request type, etc. The coherence hub 72 receivesthe node request, serializes and orders the requests with otherrequests, and transmits a forwarded request snoop (FR snoop) to thenodes 74 and 76. Additionally, the coherence hub 72 transmits a noderequest acknowledge (NR Ack) to the source node 70. The source node 70may use the NR Ack to trigger a speculative read to the local memory,for example.

Each of the nodes 74 and 76 snoop the address from the FR snoop messageand transmit node snoop reply (NSR) messages to the coherence hub 72.The NSR messages may indicate the state of the cache line in the nodes74 and 76, respectively. If a hit in the owned state or modified stateis indicated, the node 74 or 76 may subsequent respond with a node datareply message (NDR, illustrated by dashed line in FIG. 8 to indicatethat the message may or may not be transmitted dependent on the snoopedcoherence state in the node 74 or 76). The coherence hub 72 receives theNSR messages, and aggregates the response from all nodes to transmit aforwarded snoop response (FSR) to the source node 70. If data is alsoprovided, a forwarded data response (FDR) containing the data isprovided by the coherence hub 72 to the source node 70. The FDR may alsoindicate if the data is from the responding node's cache, and thus thedata may be a cache to cache transfer to the source node's L2 cache.

Either the FSR or the FDR messages may include an indication that theSID for the request is being released, and thus may be reused foranother request by the source node 70. Since the SID release is includedin another message, the these mechanisms provide an implicit release ofthe SID. Bandwidth may be conserved if the SID can be released via anindication in another message, and in some cases an early release of theSID message may permit the SID to be reused earlier than would beotherwise possible. For example, if the FSR message indicates no hitsfor an address that is local to the source node 70, the FSR is the lastmessage transmitted by the coherence hub and the SID release may beincluded in the FSR. If the FDR message is the last message transmitted,the SID release may be included in the FDR message. However, in somecases, it may not be possible to release the SID in an FSR or FDRmessage. In such cases, the coherence hub may generate an explicit SIDrelease message (SidR). Thus, a flexible SID release mechanism may besupported in some embodiments.

FIG. 9 is a block diagram illustrating communications according to oneembodiment of the coherence protocol and coherence messages. In theexample of FIG. 9, the source node is not the memory agent for theaddress affected by the coherency activity. That is, the address isremote to the source node. Particularly, the node 74 is the memory agentfor the address in this example. The example of FIG. 9 may representcoherence activities for various read requests (e.g. read to share, readto own, read to discard, etc.) and a flush message. While 3 nodes areshown in FIG. 9, any number of two or more nodes may be used in variousembodiments.

Similar to FIG. 8, the source node 70 issues a node request to thecoherence hub 72, which transmits FR snoops to nodes 74 and 76 and an NRAck to the source node 70. Node 74 responds with an NSR message thatindicates that node 74 is the memory agent for the address, and alsoprovides an NDR with the data from the node 74 (either from the L2 cacheor from the memory attached to the node 74). The node 76 provides an NSRmessage, and may provide an NDR with data if the NSR has the data inowned or modified state. The coherence hub 72 transmits the FSR messageto the source node 70, aggregating the NSRs from the nodes 74 and 76. Ifthe node 76 responds with data, that data may supersede the data fromthe node 74 (as it may be more recent than the data from the node 74)and may be provided in the FDR to the source node 70. Otherwise, thedata provided by the node 74 is provided in the FDR to the source node70. Optionally, the SidR message may be transmitted if the SID is notreleased in the FSR or FDR message.

In some embodiments, a node request may be made to upgrade ownership ofa cache line in a source node so that an access may be completed withinthe source node. For example, an invalidate request may be used toupgrade a shared or owned state to modified to complete a write request.Such requests may operate similar to FIGS. 8 and 9, except that no datamay be transmitted.

FIG. 10 is a block diagram illustrating communications according to oneembodiment of the coherence protocol and coherence messages. In theexample of FIG. 10, the source node performs a writeback to the memoryagent for the address. Particularly, the node 74 is the memory agent forthe address in this example. The node 76 is also shown, although it isnot involved in the writeback operation.

The source node 70 transmits a node request for the writeback (WB) tothe coherence hub 72. The coherence hub 72 transmits the NR Ack to thesource node 70, and the forwarded request WB (FR WB) to the node 74,which is the memory agent for the address. The source node 70 transmitsthe node data (ND) to the coherency hub 72, which forwards the data as aforwarded data (FD) message to the node 74. Additionally, the coherencehub 72 may transmit the SidR message to the source node 70 to free theSID assigned to the writeback.

Non-cacheable requests and I/O requests (e.g. from the I/O control unit20) may be performed without coherence activity, and thus may be similarto the communication shown in FIG. 10. For non-cacheable or I/O reads,the direction of data flow is the opposite of that shown in FIG. 10.

Turning now to FIG. 11, a block diagram of one embodiment of an FBDIMMlink is shown, illustrating the transmission of coherence messages onthe FBDIMM link. Frame boundaries on the link are illustrated in FIG. 11as vertical dashed lines. Accordingly, two frames 80 and 82 are shown,and a portion of a frame 84 is shown.

FIG. 11 illustrates that, although the physical layer and framing of theFBDIMM interface are used for the coherent links between nodes, thecoherence messages may not be bound to the frames. For example, two ormore coherence messages may be transmitted within the same frame. Inframe 80 in FIG. 11, two coherence messages (messages 0 and 1) aretransmitted. Additionally, coherence messages may straddle a frameboundary. For example, message 3 in FIG. 11 is partially transmitted inframe 82 and partially in frame 84.

Coherence messages may generally have any format. In the illustratedembodiment, the coherence messages may each include a type field (T), anSID field (sid), and other information (O). The other information may bemessage-specific, and may include information such as one or more of theaddress, the data, a request field identifying which specific request orresponse is transmitted, a destination node identifier, various statusbits, etc. Different coherence message types may have different lengths.The bandwidth may be efficiently used by, for example, defining messagetypes that carry less information to be shorter than other messagetypes. The types may be, e.g., request, response, and data. The SIDfield carries the SID of the message.

In some embodiments, the coherence links may have added features overthe physical FBDIMM link and framing. For example, the coherence linksmay implement reliable delivery. The transmitter on a link may retain atransmitted coherence message and, if an error occurs at the receiverfor the message, the transmitter may retransmit the message. In somecases, a more robust cyclical redundancy check (CRC) or other errordetection mechanism may be implemented for the coherence messages thanthe frame-based error detection that may be provided on the FBDIMMlinks.

Turning now to FIG. 12, a flowchart is shown illustrating one embodimentof a method for maintaining coherency in a system using coherenceplanes. While the blocks shown are illustrated in a particular order forease of understanding, other orders may be used. Blocks may beimplemented in parallel in combinatorial logic circuitry. Blocks,combinations of blocks, or the flowchart as a whole may be pipelinedover multiple clock cycles.

The address for the request may be generated in the source node (block90). Particularly, for example, one of the processor cores 12A-12N maygenerate the address. The address may miss in the L1 cache(s) in theprocessor core, and may be transmitted to the L2 cache 14. If nocoherence activity external to the node is needed to complete therequest (decision block 92, “no” leg), the L2 cache 14 may complete therequest (block 94). If coherence activity external to the node is needed(decision block 92, “yes” leg), the address may be mapped to one of thecoherence planes (block 96), and the coherence activity may be performedon the coherence plane (block 98). When the coherence activity iscompleted, the request may be completed (block 94). In some embodimentsabove, mapping the address to a coherence plane may be implicit inmapping the address to an L2 cache bank. Other mappings of addresses tocoherence planes may be used in other embodiments.

Turning now to FIG. 13, a block diagram of another embodiment of asystem including the CMT nodes 10A-10B is shown. In this embodiment, theinterfaces 24 between the nodes are used both for communicatingcoherence messages and for accessing memory. For example, in theillustrated embodiment, the interfaces are coupled to one or moredual-inline memory modules (DIMMs) 100A-100Q. Any number of DIMMs may beincluded, or other types of memory modules may be included. In theillustrated embodiment, the interfaces 24 comprise point to point linksand thus the DIMMs 100A-100Q are coupled in a daisy chain fashionbetween the nodes 10A-10B. Particularly, the interfaces 24 may be FBDIMMinterfaces and the DIMMs 100A-100Q may be FBDIMMs. FBDIMMs includeadvanced memory buffers (AMBs) such as AMBs 102A-102Q for bufferingcommunications from the FBDIMM interfaces and for controllingcommunication on the interfaces. As illustrated in FIG. 13, theinterface 24A is coupled between the node 10A and the DIMM 100A.Similarly, the interface 24K is coupled between the DIMMs 100A-100B, andthe interface 24L is coupled between the DIMM 100Q and the node 10B.Other interfaces 24 may be coupled between other DIMMs that may beincluded in the system (not shown in FIG. 13).

Since the DIMMs 100A-100Q are coupled to the same communication pathused to transmit coherence messages between nodes 10A-10B, the trafficon the path may comprise a mix of memory accesses and coherencemessages. The coherence units and memory control units in the nodes10A-10B may share a link interface unit (LIU) 42 to communicate on theinterfaces 24. The LIU 42 may arbitrate between the coherence units andmemory control units, or the coherence units and memory control unitsmay communicate directly to control transmission on the interfaces 24.

In the illustrated embodiment, the coherence unit 18A and the MCU 16Amay share the LIU 42 to communicate on the interface 24A. Similarly, thecoherence unit 18E and the memory control unit 16C may share the LIU 42to communicate on the interface 24L. Other coherence units 18B-18D and18F-18H may communicate over other interfaces between the nodes 10A-10B.Some or all of those interfaces may also be populated with DIMMs similarto the DIMMs 100A-100Q, and memory control units may share theinterfaces with the coherence units.

The AMBs 102A-102Q may receive a frame from an interface, and may checkthe frame to determine if it includes a memory access for thecorresponding DIMM 100A-100Q (i.e. the DIMM that includes the AMB). Ifthe frame includes a memory access for the corresponding DIMM 100A-100Q,the AMB 102A-102Q may process the frame and cause the access to one ormore memory chips on the DIMM. If the memory access is a read, the AMB102A-102Q may return the data on the same interface from which the framewas received. If the frame does not include a memory access for thecorresponding DIMM (e.g. it includes a memory access for another DIMM orit includes one or more coherence messages), the AMB 102A-102Q maypropagate the frame in the direction that the frame was traveling. Forexample, if the AMB 102A receives a frame from the interface 24A, theAMB 102A may propagate the frame on the interface 24K to the DIMM 100B.If the AMD 102A receives a frame from the interface 24K, it maypropagate the frame on the interface 24A to the node 10A.

While the illustrated embodiment populates an interconnect between twonodes with memory modules, other embodiments may populate aninterconnect between nodes and a coherency hub (e.g. similar to theembodiment of FIG. 3) in a similar fashion.

Numerous variations and modifications will become apparent to thoseskilled in the art once the above disclosure is fully appreciated. It isintended that the following claims be interpreted to embrace all suchvariations and modifications.

1. A node comprising: a plurality of processor cores, each processorcore configured to have a plurality of threads active and each processorcore including at least one first level cache; coherency controlcircuitry coupled to the plurality of processor cores and configured tomanage intranode coherency among the plurality of processor cores; andat least one coherence unit coupled to the coherency control circuitryand configured to couple to an external interface of the node, thecoherence unit configured to transmit and receive coherence messages onthe external interface to maintain coherency with at least one othernode having one or more processor cores and a coherence unit.
 2. Thenode as recited in claim 1 wherein the coherence unit comprises acoherence hub that is used if the node connects to only one other nodethrough the external interface.
 3. The node as recited in claim 3wherein the coherence hub is disabled if the node is used in a systemthat includes two or more other nodes, and wherein the externalinterface is used to couple to an external coherence hub.
 4. The node asrecited in claim 1 wherein the node comprises at least one memorycontrol unit configured to couple to a memory.
 5. The node as recited inclaim 4 wherein the at least one memory control unit comprises aplurality of memory control units each configured to couple to aseparate memory.
 6. The node as recited in claim 1 wherein the nodefurther comprises a second level cache coupled to each of the processorcores and to the coherence unit, wherein the second level cachecomprises the coherency control circuitry.
 7. A system comprising: aninterconnect; and a plurality of nodes coupled to the interconnect,wherein each of the plurality of nodes comprises: a plurality ofprocessor cores, each processor core configured to have a plurality ofthreads active and each processor core including at least one firstlevel cache; coherency control circuitry coupled to the plurality ofprocessor cores and configured to manage intranode coherency among theplurality of processor cores; and at least one coherence unit coupled tothe coherency control circuitry and configured to couple to theinterconnect, the coherence unit configured to transmit and receivecoherence messages on the interconnect to maintain coherency among theplurality of nodes.
 8. The system as recited in claim 7 wherein theinterconnect comprises a pair of point to point links to which a firstnode of the plurality of nodes is coupled and to which the second nodeof the plurality of nodes is coupled to directly communicate between thecoherence unit in the first node and the coherence unit in the secondnode.
 9. The system as recited in claim 8 wherein each of the coherenceunits in the first node and the second node comprise a coherence hub tocontrol coherence messages between the first node and the second node.10. The system as recited in claim 8 wherein a number of the pluralityof nodes is two.
 11. The system as recited in claim 7 wherein theinterconnect comprises a plurality of pairs of point to point links,wherein each of the plurality of nodes is coupled to a respective pairof the pairs of point to point links, and wherein the system furthercomprises a coherence hub external to the plurality of nodes and coupledto each of the pairs of point to point links, wherein the coherence hubis configured to route coherence messages between the plurality ofnodes.
 12. The system as recited in claim 7 wherein each of theplurality of nodes comprises at least one memory control unit configuredto couple to a memory.
 13. The system as recited in claim 12 wherein theat least one memory control unit comprises a plurality of memory controlunits each configured to couple to a separate memory.
 14. The system asrecited in claim 7 wherein each of the processor cores comprises atleast one first level cache, and wherein the node further comprises asecond level cache coupled to each of the processor cores and to thecoherence unit, wherein the second level cache is configure to controlcoherency among the first level caches in a given one of the pluralityof nodes.
 15. An apparatus comprising: a source node; and a coherencehub configured to route coherence messages between the source node andother nodes; wherein the source node is configured to generate a noderequest, which is one of the coherence messages, to initiate coherenceactivity for access to a given memory location, and wherein the sourcenode is configured to assign a source identifier to the node requestthat identifies the source node and the node request, and wherein thecoherence hub is configured to indicate that the source identifier isreleased for assignment to another node, and wherein the coherence hubis configured to provide the indication in another coherence message tothe source node that is part of the coherence activity, if possible, andwherein the coherence hub is configured to explicitly transmit a sourceidentifier release message to provide the indication if the indicationcannot be provided in another coherence message that is part of thecoherence activity.
 16. The apparatus as recited in claim 15 wherein theother coherence message in which the indication is provided includes thesource identifier.
 17. The apparatus as recited in claim 15 wherein thecoherence hub is integrated into the source node.
 18. The apparatus asrecited in claim 15 wherein the coherence hub is separate from thesource node and wherein the source node comprises a coherence unitconfigured to communicate with the coherence hub.
 19. The apparatus asrecited in claim 15 wherein the source node comprises a plurality ofprocessor cores, wherein each of the plurality of processor cores isconfigured to have a plurality of threads active.
 20. The apparatus asrecited in claim 15 wherein the source node comprises a plurality ofmemory control units, wherein each of the plurality of memory controlunits is configured to couple to a separate memory.